1. Field of the Invention
The present invention concerns a time-variable magnetic fields generator with a primary gradient coil unit for generation of a gradient magnetic field in an examination region of a magnetic resonance apparatus. Furthermore, the invention concerns a magnetic resonance apparatus with at least one such generator.
2. Description of the Prior Art
Magnetic resonance technology is a known technology to, among other things, acquire images of the inside of a body of an examination subject. Rapidly switched gradient magnetic fields generated by gradient coils are superimposed on an existing static basic magnetic field B0 that is generated by a basic field magnet in an examination region of the magnetic resonance apparatus (MR apparatus). To excite magnetic resonance signals, the magnetic resonance apparatus also has a radio frequency antenna (RF antenna) that radiates RF signals (known as the B1 field) into the examination subject. The resulting magnetic resonance signals can be acquired using this antenna or further local antennas and are processed into magnetic resonance images. The B1 field generally is generated by currents through a conductor.
For example, an open magnetic resonance apparatus with a C-shaped basic field magnet is known from U.S. Pat. No. 6,433,550. A patient volume of the magnetic resonance apparatus, in which the basic magnetic field of the magnetic resonance apparatus is generated in an optimally homogeneous manner, is disposed in the opening of the C-shaped basic field magnet. Furthermore, two pole plates are disposed at the two ends of the C-structure on which are disposed parts of a gradient coil system fashioned essentially planar in the direction towards the patient volume. As disposed at these locations are parts of an antenna system of the magnetic resonance apparatus, which are likewise fashioned essentially planar. Gradient coils for the aforementioned gradient coil system are described in detail in German OS 40 37 894 and German OS 19544779. Actively shielded gradient coils for MR apparatuses are, for example, known from German PS 44 22 781 and German PS 44 22 782.
An antenna arrangement for a magnetic resonance apparatus in which a basic magnetic field is established between two pole plates is also known from German OS 42 32 884. A sub-antenna into which a radio frequency signal is fed is arranged at each pole plate. Each sub-antennas is formed by a closed, grounded shield facing the respective pole plate and a laminar (think, flat) conductor structure disposed at a distance from the shield and essentially parallel thereto. Furthermore, from German OS 42 32 884 ills known that with basic field magnets formed by pole plates it is attempted to keep the pole plate separation optimally small so that the weight of the basic field magnet remains small and a better basic magnetic field homogeneity is achieved. For this reason, it is advantageous to keep all internal components between the pole plates (such as the gradient coil system, the radio frequency shield and the antenna arrangement) as thin as possible. This competes with the fact that an optimally large separation of the antenna arrangement from the radio frequency shield is advantageous for a higher degree of efficiency of the specified antenna arrangement. This is particularly true given the use of the antenna arrangement to receive magnetic resonance signals.
A circular, planar radio frequency antenna for open magnetic resonance apparatuses is known from German OS 101 24 737. It has two separated systems made of planar conductors arranged on a carrier plate for intercrossing currents. For tuning to the desired resolution frequency, the conductors are capacitively shorted on at least one end by tuning capacitors connected to ground. Only one planar metal layer is arranged on each carrier plate, into which metal layer both currents, phase-offset by 90°, are fed. Tuning capacitors are arranged at the feed location and the opposite side.
Generally in magnetic resonance systems the dimension of the volume within which the homogeneous basic magnetic field B0 is homocieneous, i.e. the pole plate separation, are a primary factor in the overall apparatus cost and therefore this volume is not made any larger than necessary. Thus measures to minimally occupy the available space with internal components or to maximally use the available space for the patient for comfortable examination are advantageous.